The present invention relates, in general, to electrical circuits that provide reference voltages and, in particular, to bandgap reference voltage circuits that are substantially insensitive to temperature and avoid undesirable metastable operation.
Bandgap reference voltage circuits are used in Bipolar and BICMOS circuit designs for producing stable reference voltages for biasing circuits. The stable reference voltages are used to control other voltage levels within a chip and to provide bias currents that are proportional to absolute temperature.
Circuits that regulate voltage and provide bias current are used extensively in analog units such as cellular telephones. A bandgap reference voltage circuit in a cellular telephone must not only provide the required voltage regulation and bias current, but also must be power efficient because cellular telephone circuits are powered by batteries. The bandgap reference voltage circuit is integral to the operation of the cellular telephone circuits, so reliability of the bandgap reference voltage circuit is essential to avoid catastrophic failure.
Normally, bandgap reference voltage circuits have two operating states. The first operating state provides normal operation that produces the required regulated voltage or bias current. The second operating state is a zero current state, which means that the bandgap reference voltage circuit is not operational. Some bandgap reference voltage circuits have a third operating state which is the metastable state representing a circuit failure.
One of the more common problems with bandgap reference voltage circuits is the failure of the circuit to enter the normal operational state from the zero state. If the bandgap reference voltage circuit enters the metastable state, the output voltage does not attain a final reference value and the circuit might remain in the metastable state, with the result that the entire cellular telephone unit might fail.
A solution to this problem is to provide additional start-up circuitry that forces the bandgap reference voltage circuit into its normal operating state (i.e., the first operating state identified above). Additional start-up circuitry, however, adds load to the battery power supply and this can decrease power efficiency.
For silicon-based technology, the bandgap voltage typically is 1.2V. This bandgap voltage also is used to generate a proportional to absolute temperature (PTAT) current for analog circuits. Sometimes, a reference voltage greater or less than 1.2V is desired to adjust the temperature coefficient of the PTAT current. This facility is important in optimizing circuit performance.
Another consideration in the design of bandgap reference voltage circuits is the current trend to reduce the supply voltage in battery operated units. As the supply voltage is reduced, for example, from 3V to 2.2V, the bandgap reference voltage circuits are likely to not function properly with the result that the bandgap reference voltages will decrease with the decreases in the supply voltage.
To overcome the shortcomings of prior art bandgap reference voltage circuit a new and improved bandgap reference voltage circuit is provided by the present invention.
It is an objective of the present invention to provide a new and improved bandgap reference voltage circuit.
It is another objective of the present invention to provide a bandgap reference voltage circuit that minimizes greatly metastable operation.
It is a further objective of the present invention to provide a bandgap reference voltage circuit that can operate properly with reductions in the supply voltage.
It is yet another objective of the present invention to provide a bandgap reference voltage circuit that develops reference voltages greater and less than the bandgap reference voltage.
A bandgap reference voltage circuit, constructed in accordance with the present invention, includes a bandgap start-up circuit for initiating operation of the bandgap reference voltage circuit, a bandgap core circuit for developing a bandgap reference voltage, and a bandgap output circuit for supplying a bandgap reference voltage. The bandgap core circuit has a low impedance leg to which bandgap start-up circuit is connected and a high impedance leg to which the bandgap output circuit is connected. The bandgap output circuit has a feedback circuit connected to the high impedance leg of the bandgap core circuit.
It is to be understood that the foregoing general description of the present invention and the following detailed description of the present invention are exemplary, but are not restrictive of the invention.